Lost Core Process
Over the last decade, a process called "Lost Core" technology has evolved in the plastics injection moulding industry. The lost core process allows the manufacturer of hollow structures, which could previously not be moulded by use of retractable or collapsible moulds, or any other types of moulds with permanent core sections. The lost core process is used to manufacture such complex parts as water pump housings, automotive engine intake manifolds, and wheels for ultralight bicycles. The lost core process allows for the manufacture of high quality surfaces on the inside of the parts, while permitting close wall thickness tolerances to be maintained.
Blow moulding processes cannot produce complex shapes as compared with those which can be produced by the lost core process. Blow moulding cannot adequately maintain wall thicknesses or produce as high quality surfaces on the inside of the parts.
Other process are known which produce complex parts from a greater number of separate moulded sections, and, in a secondary operation, join them together, via ultrasonic or frictional welding, adhesive bonding, or other methods. Such processes have the disadvantages that the resultant inner surface of the part is disturbed by the joints and that properties of the part, such as impact resistance are frequently inferior. As well, such processes have the disadvantages associated with moulding a greater number of parts and handling them.
The "lost core" manufacturing process involves a number of different steps. First, a core, representing the exact inner geometry of the plastic part to be manufactured, must be casted or moulded. In most cases, a Sn-Bismuth alloy is used having a low melt temperature in the range of 100.degree. C. and 200.degree. C. Known alloys are preferred which do not show any significant thermal expansion or contraction in the range of use, that is from room temperature of about 20.degree. C. to around 200.degree. C., since their use makes mould design much simpler. Preferred such alloys do not exhibit greater than 1% expansion in this temperature range. Specific grade Sn-Bismuth alloys are preferred having a melting point in the range of 135.degree. C. to 140.degree. C. preferably 138.degree. C. due to their minimal thermal expansion.
The casting of the lost core takes place in a closed mould, into which the tin-Bismuth alloy is pumped. Under normal circumstances a low pressure such as 6 bar is sufficient for such casting, resulting in relatively low clamping forces. After the lost core has cooled down somewhat and the surface of the core has solidified sufficiently that the core can be handled, the mould is opened and the core is removed. The core is then placed on a cooling conveyor to further cool down.
At the end of this cooling conveyor, the core is picked up and inserted into a plastic injection mould, where thermoplastic or thermoset materials are moulded over the core. After a desired cooling time, the plastic part with the core inside is removed from the mould. Next, the core is melted out from within the part such that the melted core will flow in a molden state out of the hollow part. After the core has flowed out, the part is washed to minimize contamination of the part as by either particles of the core or compounds used to assist in removal of the core remaining on the part. The part is now ready for inspection and further assembly.
There are two basic known methods to melt the core from the plastic part, namely, a glycol bath method and an induction heat method.
Glycol Bath Method
In the glycol bath method, the part including the core are immersed in a glycol based liquid, which is heated, in most cases to 175.degree. C. The residence time of the part in the gycol bath is typically approximately one hour, depending the size and volume of the core. To accommodate high volume production, such melt out baths are typically large, and particularly contain hanging conveyors, on which the parts are mounted and conveyed.
The size of typical meltout baths requires the use of large quantities of the glycol based liquids. These substances as typified by a product sold under the trade name LUTRON by BASF and are expensive. In normal use, the LUTRON liquid must be replaced every 12 months even when maintenance procedures are carefully followed. Contaminated LUTRON liquid is expensive to dispose of after its useful life.
The large size of the glycol bath has the disadvantage of significant heat losses in use, which losses reduce the energy efficiency of the glycol bath method.
With the glycol bath system, when the plastic parts emerge from the bath with the core removed, they are transported to a washing station. On the way to the washing station, some of the glycol liquid drips off the parts, and this creates collection and containment problems as well as resulting in a loss of the glycol liquid.
The purpose of the washing station is to remove from the part all remaining glycol liquid and particles of the core alloy which may remain on the part. For this purpose, water is used. Since the glycol liquid has a specific gravity, very close to that of water, it is difficult to separate glycol from water. A distillation process is employed to recover the glycol from the wash water, which process is a high energy consuming process. Removal of the glycol from the water is necessary since wash water, contaminated with glycol, cannot, environmentally or lawfully be discarded into a sewer system.
Induction Heat Method
In the induction heat method to melt out the core, the parts including the cores are removed from the plastic injection moulding machine, and then dipped into a small tank, again containing the same glycol solution as described above. However, in this tank, there is also an induction coil, with a shape similar to the outer shape of the moulded part. As is known by passing electric current through the induction coil, an electrical field is developed in the tank passing through the part and as a result of which, the metal core is heated. The core is heated until the core liquefies, and flows out of the plastic part.
The induction heat process is relatively fast and typically the metallic core may be melted out in just a few minutes, allowing for a much smaller physical size of the tank. However, the induction heat system has some major disadvantages. Firstly, the expensive glycol based liquid needs to be used in the bath so as to reduce temperature peaks within the plastic shell during the melt out operation. Secondly, the power consumption of such an induction system is very high due to the low efficiency of such induction heating of the core. Thirdly, localised temperature peaks can lead to damage of the plastic material, from which the part is moulded, and to faster degradation of the expensive glycol liquid.
One of the major drawbacks of the induction heat method is the life time of the induction coils. The tin-Bismuth alloy used as the core has the tendency to creep into the smallest crack or crevice and, being a very good electrical conductor, leads to short circuits of the coils resulting in the need to frequently replace the coils. Not only are coils expensive but the downtime of the system to replace coils represents a large loss in productivity.